Extraordinary branching ratios in astrophysically important dissociative recombination reactions

Abstract

Branching ratios of the dissociative recombination reactions of the astrophysically relevant ions DCO⁺, N₂H⁺ and DOCO+ (as substitute for HOCO⁺) have been measured using the CRYRING storage ring at the Manne Siegbahn Laboratory at the University of Stockholm, Sweden. For DCO⁺, the channel leading to D and CO was by far the most important one (branching ratio 0.88), only small contributions of the CD + O and OD + C product pathways (branching ratios 0.06 each) were recorded. In the case of N₂H⁺ the surprising result of a break-up of the N–N bond to N and NH (branching ratio 0.64) was found with the branching ratio of the N₂ + H product channel therefore displaying a branching ratio of only 0.36. In the case of DOCO⁺, the three-body break-up into D + O + CO dominated (branching ratio 0.68), whereas the contribution of the CO₂ + H channel was only minute (0.05). The remaining share (branching ratio 0.27) was taken by the pathway leading to OH + CO. For the dissociative recombination of N₂H⁺ and DOCO+ also absolute reaction cross sections were obtained in the collisional energy range between 0 and 1 eV. From these cross sections it was possible to work out the thermal rate constants, which were found to be k(T) = 1.0 ± 0.1 × 10⁻⁷ (T/300 K)^{−0.51±0.02} cm³ s⁻¹ and k(T) = 1.2 ± 0.1 × 10⁻⁶ (T/300 K)^{−0.64±0.02} cm³ s⁻¹ for N₂H⁺ and DOCO⁺, respectively.